Generally, as the production method of a polyimide, there is adopted a method including: synthesizing a polyamic acid which is an imide precursor by reacting a tetracarboxylic dianhydride and a diamine as monomers; and imidizing the polyamic acid by using either (a) a thermal imidization method for cyclizing by heating and dehydrating or (b) a chemical imidization method for cyclizing using a dehydrocondensing agent and an imidization catalyst.
The (a) thermal imidization method is a method for imidization in which a heating-dehydration reaction is effected by the use of an acid catalyst and an azeotropic solvent as assistants for the imidization reaction. At this time, generally, the polyamic acid is heated to a temperature of 180° C. to 250° C. to effect the reaction.
In contrast, as the (b) chemical imidization method, there is known a method in which a dehydrocondensing agent and a ring-closure catalyst are directly added to a polyamic acid solution to heat and dry the resultant mixture. As the dehydrocondensing agent, there are known carboxylic anhydrides such as acetic anhydride, propionic anhydride and benzoic anhydride, and dicyclohexylcarbodiimide (DDC). As the ring-closure catalyst, there are known aliphatic tertiary amines such as triethylamine, and heterocyclic tertiary amines such as isoquinoline, pyridine, β-picoline, aminopyridine and imidazole, and particularly, an acetic anhydride-pyridine type is widely used (for example, Patent Document 1).
On the other hand, a polyimide resin is excellent in heat resistance, electric insulation, solvent resistance and mechanical stability, and is used in various fields.
Particularly, in a field of electric and electronic materials, it has been found that a cured film produced from a photosensitive polyhydroxyimide-based resin to which photosensitive properties are imparted by introducing a hydroxy group into a polyimide possesses not only the above characteristics of a polyimide-based resin, but also high resolution and high sensitivity. Therefore, the application of such a cured film has been expanded and starts to prevail not only in a semiconductor field, but also in a display field.
For example, as a material capable of obtaining a positive pattern having high resolution, high sensitivity and advantageous dimension controllability, there is disclosed a positive photosensitive resin composition using a polyhydroxyimide (Patent Document 2).
A polyhydroxyimide can be obtained from a precursor polyhydroxyamic acid by the (a) thermal imidization method or the (b) chemical imidization method. However, actually, in the (b) chemical imidization method, even a hydroxy group in side chains of a polyamic acid is reacted with a large amount of acetic anhydride existing within the reaction system to be converted into an acetoxy group. This means that when a polyhydroxyimide is applied in a photosensitive resin composition or the like, a necessary developing group (hydroxy group) becomes deactivated.
Therefore, there is disclosed a method including: synthesizing an imide precursor in, for example, N-methyl-2-pyrrolidone (NMP): adding an aromatic hydrocarbon solvent (such as m-xylene) which azeotropically boils with water as a dehydrating agent to the imide precursor; and heating the resultant mixture to 180° C. to subject the imide precursor to a dehydration reaction to imidize the imide precursor (Patent Document 2).
As another method, there is also disclosed a method for directly producing a polyhydroxyimide, for example, by heating an acid dianhydride and a diamine at a high temperature (180° C.) in the presence of an acid catalyst (γ-valerolactone) and a base (pyridine) in a solvent mixture of an aprotic polar solvent (such as NMP) and a dehydrating solvent (such as toluene) to progress the reaction while azeotropically removing a water content (Patent Document 3).    Patent Document 1: Japanese Patent Application Publication No. JP-A-2-151629    Patent Document 2: Japanese Patent Application Publication No. JP-A-2005-173027    Patent Document 3: Japanese Patent Application Publication No. JP-A-2002-212287